Sub-nanometer scale depth patterning on sapphire crystal by femtosecond soft x-ray laser pulse irradiation.

Damage thresholds and structures on a metal aluminum and an aluminum oxide crystal induced by the soft x-ray free electron laser irradiations were evaluated. Distinctive differences in damage thresholds and structures were observed for these materials. On the aluminum oxide crystal surface, in particular, a novel, to the best of our knowledge, surface processing, which we suggest defining as "peeling," was recognized. Surface structures formed by peeling had extremely shallow patterning of sub-nanometer depth. For the newly observed peeling process, we proposed a scission of chemical bond, i.e., binding energy model, in the crystal.

Soft X-ray ptychography system using a Wolter mirror for achromatic illumination optics.

A soft X-ray ptychography system using a Wolter mirror for the illumination optics has been developed. By taking advantage of the achromaticity of the optics, the system is capable of seamlessly imaging at half-period resolution of 50 nm with a broad photon-energy range from 250 eV to 2 keV while maintaining the focal position. Imaging a mammalian cell at various wavelengths was demonstrated, and high-resolution visualization of organelle was achieved. Stereo imaging was also performed with a long working distance of 20 mm. In combination with in-situ/operando and tomographic measurements, this system will be a powerful tool for observing biological and material targets with complex features.

Intense sub-micrometre focusing of soft X-ray free-electron laser beyond 1016 W†cm-2 with an ellipsoidal mirror.

Intense sub-micrometre focusing of a soft X-ray free-electron laser (FEL) was achieved by using an ellipsoidal mirror with a high numerical aperture. A hybrid focusing system in combination with a Kirkpatrick-Baez mirror was applied for compensation of a small spatial acceptance of the ellipsoidal mirror. With this system, the soft X-ray FEL pulses were focused down to 480 nm × 680 nm with an extremely high intensity of 8.8×1016 W cm-2 at a photon energy of 120 eV, which yielded saturable absorption at the L-edge of Si (99.8 eV) with a drastic increase of transmittance from 8% to 48%.

Full-field microscope with twin Wolter mirrors for soft X-ray free-electron lasers.

We developed a full-field microscope with twin Wolter mirrors for soft X-ray free-electron lasers. The Wolter mirrors for a condenser and an objective were fabricated using an electroforming process with a precisely figured master mandrel. In the imaging system constructed at SACLA BL1, sub-micrometer spatial resolution was achieved at wavelengths of 10.3 and 3.4 nm. Single-shot bright-field images were acquired with a maximum illumination intensity of 7×1014 W/cm2.

Efficient and precise fabrication of Wolter type-I x-ray mirrors via nickel electroforming replication using quartz glass mandrels.

This study presents an approach for fabricating Wolter type-I mirrors for x-ray telescopes using a nickel electroforming replication process with quartz glass mandrels. The proposed method addresses the challenges encountered in conventional fabrication techniques, which involve using electroless nickel-coated aluminum mandrels that are susceptible to corrosion and thermal deformation. Quartz glass mandrels offer excellent chemical, thermal, and mechanical stability, enabling the efficient production of high-performance mirrors. Wolter type-I mirrors for telescopes possess a large aperture that collects x-ray photons from the universe. However, previous nickel electroforming replication processes using quartz glass mandrels have challenges in fabricating large mirrors, particularly due to bubble pit formation during nickel shell development. In this study, we introduced an efficient pitting inhibition technique via vacuum degassing. This technique facilitates the precise replication of pit-free Wolter type-I mirrors for telescopes using quartz glass mandrels. We demonstrated the fabrication process on a Wolter type-I mirror proposed for FOXSI-4 [(FOXSI) Focusing Optics X-ray Solar Imager], resulting in three mirrors obtained from the same mandrel without repolishing or repairing. The figure error of the mirror was within 1 µm over most areas in both longitudinal and circumferential directions. The ray-tracing simulation indicated that the performance of the mirror was ∼12 arcsec in half-power diameter, comparable to the performance achieved by previous high-resolution x-ray missions.

Figure correction of a Wolter mirror master mandrel by organic abrasive machining.

In this study, figure correction of a master mandrel of a Wolter mirror by organic abrasive machining (OAM) was demonstrated. In OAM, a flow of slurry, dispersed with organic particles, locally removes the surface of a workpiece in contact with a rotating machining tool. A computer-controlled machining system was used to perform the selective removal of a fused silica surface at a spatial resolution of 200 µm. A master mandrel of a Wolter mirror for soft x-ray microscopes was fabricated with a figure accuracy of <1 nm root mean square, which is sufficient for diffraction-limited imaging at a wavelength of 10 nm.

Copper electroforming replication process for soft x-ray mirrors.

We developed a copper electroforming replication (CER) process to fabricate precise ellipsoidal mirrors for soft x-ray focusing. Some applications of ellipsoidal mirrors in x-ray microscopy require that all components that are close to samples, including the mirrors, are made of non-magnetic materials. In this study, a non-magnetic copper ellipsoidal mirror was fabricated by replicating a figured and super-polished quartz glass mandrel using an electroforming technique. It was found that the CER process has a high replication accuracy of 8 nm. The focusing performance of the mirror was characterized using a soft x-ray free-electron laser with a photon energy of 100 eV. A small focus size of 370 × 400 nm2 was achieved with a high reflectivity of 65%.

Development of electroforming process for soft x-ray ellipsoidal mirror.

An x-ray ellipsoidal mirror is an ideal tool for focusing soft x-rays. Because nanometer-level shape accuracy is required in the internal surface of a mirror having a small diameter, it is difficult to fabricate the mirror by processing the surface directly. We developed a fabrication process for soft x-ray ellipsoidal mirrors in which the surface of a high-precision quartz mandrel with the inverted shape of the designed mirror is replicated by nickel sulfamate electroforming. In this study, an ellipsoidal mirror of 40-mm length was fabricated and the shape accuracy of the replicated surface was evaluated by a measurement method using a contact probe. The root mean square (RMS) of the replication error in the entire measured surface was 27.2 nm. When the evaluated area was half the replicated surface near the middle of the mirror, the RMS of the replication error was 14.7 nm. Wave-optical simulation suggested that it is possible to focus soft x-rays to a spot with a diameter of 400 nm.

Fabrication of a precise ellipsoidal mirror for soft X-ray nanofocusing.

In X-ray focusing, grazing incidence mirrors offer advantages of no chromatic aberration and high focusing efficiency. Although nanofocusing mirrors have been developed for the hard X-ray region, there is no mirror with nanofocusing performance in the soft X-ray region. Designing a system with the ability to focus to a beam size smaller than 100 nm at an X-ray energy of less than 1 keV requires a numerical aperture larger than 0.01. This leads to difficulties in the fabrication of a soft X-ray focusing mirror with high accuracy. Ellipsoidal mirrors enable soft X-ray focusing with a high numerical aperture. In this study, we report a production process for ellipsoidal mirrors involving mandrel fabrication and replication processes. The fabricated ellipsoidal mirror was assessed under partial illumination conditions at the soft X-ray beamline (BL25SU) of SPring-8. A focal spot size of less than 250 nm was confirmed at 300 eV. The focusing tests indicated that the proposed fabrication process is promising for X-ray mirrors that have the form of a solid of revolution, including Wolter mirrors.